This invention relates to a pressure relief valve. More particularly, the invention relates to a pressure relief valve of the type having a pressure responsive poppet-type valve member that opens at a predetermined pressure to relieve pressure of a working fluid in a device.
Pressure relief valves of this general type are well known and the art is replete with different designs for different applications. One application, and one for which the present invention is particularly suited, is for maintaining or limiting the maximum pressure of a compressible fluid, such as air, to a device operated by or otherwise requiring a supply of pressurized compressible fluid. Typical applications would include any device incorporating an air motor or air actuated mechanism such as for example, air tools including impact wrenches, grinders, nailers and staplers among others. Such devices can be destroyed or damaged when subjected to pressures exceeding design limits and it is typical to provide pressure relief valves of the general type to which the invention relates to limit the pressure of the fluid supplied to the device to a maximum permissible safe level.
Despite the inclusion of such pressure relief valves, it is possible with some tools to raise the supply pressure above the maximum permissible level to such an extent that the pressure delivered to the operating mechanism will also be raised above the permissible pressure level even though the relief valve has functioned in its intended manner to vent the excess pressure. While the device may operate at the elevated pressure, sustained operation at the elevated pressure can eventually damage the device. Accordingly, it would be advantageous, and desirable, particularly for a manufacturer or supplier of the device, to have a permanent indication as to whether the device has been subjected to unreasonably high pressures. Such permanent indication that the device was subjected to pressures outside of its design limits could be used for example as one criteria for evaluating the validity of warrantee claims in case of failure or damage to the device.